This invention relates to a master cylinder assembly, primarily for a vehicle hydraulic braking system, and of the general type having a plurality of pressure cylinders between which extends a transfer passage controlled by valve means to permit communication between respective pressure chambers of the cylinders when the cylinders are actuated simultaneously and to prevent such communication when one of the cylinders is operated individually. Such an assembly is of particular use in an agricultural tractor, for example, in which normal straight-line braking is accomplished by the simultaneous actuation of a pair of cylinders and steering assistance can be provided by actuating one or the other of the cylinders individually.
Master cylinder assemblies of the aforesaid general type are known in which the valve means are actuated mechanically, often by the pressure pistons engaging valve elements during part of the piston travel. Although satisfactory in use, such arrangements are subject to the inevitable wear suffered by the mechanical components and often require special shaping of the pistons to achieve the desired operational function which adds to manufacturing costs. One proposal which avoids the use of mechanically operated valves employs a hydraulically operated compensating piston subject on opposed faces respectively to the pressures of a pair of cylinders. The compensation available by this means depends on the available movement of the compensating piston, which can be insufficient in some practical arrangements. The provision of such a compensating piston is also relatively expensive.